The present invention pertains to a method for preparing a sample for surface analysis in ultrahigh vacuum surface analysis instruments and to apparatus for accomplishing such preparation.
In the scientific community, as well as in industry, it often is desirable to obtain a chemical or physical analysis of the surface of a sample of material. Various surface analysis instruments have been developed which employ a high vacuum environment in which the sample remains during the analysis. One such surface analysis instrument is known as the VG Escalab MKII sold through VG Instruments Inc., Stamford, Conn.
Many materials desired to be surface-analyzed at ambient temperatures are liquid or, when solid, contain components which are liquids (for example, oils, compounded plastics and rubbers, latices, biological samples, etc.). If samples of such materials are subjected to a high vacuum at ambient temperature, the liquid materials in practically all instances will flash vaporize with resultant massive instant contamination of the analysis system, while any liquid components contained in a multi-component, apparently solid, material would diffuse to the surface of the sample to produce erroneous surface analysis results and ultimately outgas into the vacuum environment with resultant contamination of the analysis system. The only practical way of preventing the aforesaid difficulties is to immobilize any potentially mobile material in the sample. One way of accomplishing this desired result would be to reduce the temperature of the sample to a temperature below the glass transition temperature (Tg) of all of the substances contained in the sample to be surfaced-analyzed, desirably to a temperature of -110.degree. C. or lower. Such reduction in temperature must be done before the sample is exposed to the high vacuum at which the surface-analysis will be performed. However, if the temperature of a sample is reduced sufficiently to immobilize its component materials in a normal atmosphere, moisture in the atmosphere will condense as frost on the surface of the sample. Introduction of the frost covered sample into a high vacuum environment of the surface analysis instrument, even if such environment also is cooled to a temperature at which the components of the sample are immobilized, will not permit a proper analysis of the surface of the sample material, since the analysis will only indicate the presence of a water surface layer (i.e., the frost deposit). In order to avoid the formation of frost on the surface of a sample to be surface-analyzed in a high vacuum environment, the atmosphere surrounding the sample must contain under 0.1 part per million (ppm) of water in the form of moisture (desirably below 0.01 ppm of water) when the temperature of the sample is lowered. Since it is not possible to remove all moisture from an atmosphere, it also is important to cool the sample to the desired temperature (desirably to -110.degree. C. or below) as quickly as possible (desirably within 10 to 15 minutes) and then to admit the sample immediately into the high vacuum surface analysis instrument for analysis. Equipment which presently is available has not been found to be able to cool a sample of material containing volatiles to the desired low temperature before the sample's introduction into the high vacuum environment for surface analysis without the formation of objectionable frost on the surface of the sample.